The present invention relates generally to a ground plane and antenna system, and more specifically, to a choke-slot ground plane and monopole antenna system.
Monopole antennas are one of the simplest forms of antennas having a single radiating element. The conventional monopole antenna is typically an electrically small antenna, which has physically small or compact dimensions. The monopole operating without a ground plane produces a substantially toroidal radiation pattern. In contrast, the monopole antenna having a ground plane produces a toroidal radiation pattern which is sliced horizontally in the direction of the surface of the ground plane. A monopole antenna with a ground plane is commonly used for long range communications, commercial broadcasting, and mobile communications.
Ideally, the monopole antenna with an infinite ground plane, in combination, produces a smooth half-toroidal radiation pattern. In practice, antennas of this type have finite ground planes which produce a radiation pattern that is uneven, scalloped and has electrical radiation below the ground plane. The scalloped pattern generated by the conventional ground plane and monopole antenna systems may vary erratically, result in a loss of gain, and experience excess spurious energy losses. In some applications, suppression of the radiation pattern below the ground plane is crucial. For example, a military jamming device radiating large amounts of power requires that the amplifying transmitter is isolated from the radiation of the antenna in order to prevent interference. In present day high power jammers, isolation levels of 120 dB are usually required. Conventional monopole antennas having a ground plane are ineffective in this application due to the spurious energy radiated below the ground plane. Additional and expensive isolation equipment is required to isolate the transmitting section from the spurious energy.
For mobile communications in the continental United States, the monopole antenna should have a high sensitivity for satellites in geosynchronous orbit. A candidate antenna for this type of communication should have a radiation pattern with a peak in sensitivity at about 45.degree. above the horizon. Conventional monopole antennas having a ground plane may have to be moved or scanned in order to find the peak sensitivity, which may occur at measured angles of 18.degree. above the horizon. The uncertain and scalloped nature of the conventional monopole antenna having a ground plane yields uncertainty as to whether poor communication is due to external interference or simple problems such as antenna alignment.
What is needed is a monopole antenna and ground plane configuration which yields a smooth radiation pattern having a high pointing angle and a high isolation from spurious radiation energy below the ground plane.
The preferred embodiments of the invention disclose an antenna system having a monopole antenna, a ground plate with a plurality of concentric annular grooves, and an adjustable feed section for movably attaching the monopole to the ground plate.
A second embodiment of the invention discloses an antenna system having a monopole antenna, a ground plate with a plurality of grooves filled with a dielectric material, and an adjustable feed section. The second embodiment yields a ground plate which is smaller in size than the comparable ground plate without dielectric material.
A third embodiment of the invention discloses a choke-slot ground plane and antenna system in which the ground plate has a frusto-conical shape. The frusto-conical shape of the ground plate is useful for allowing a different range or `window` of available angles of peak sensitivity which can vary as a function of the height of the adjustable feed section.